Why couldn't Tirek use Discord's chaos magic? He's used it before in "Twilight's Kingdom". However, at that point in "Twilight's Kingdom", Tirek was stronger than he was here — the bell returned him to his third form, but he took Discord's magic once he was in his fourth form, having just absorbed the magic of the Mane Six. It's likely that in his current state Tirek isn't strong enough to control Discord's magic like he could before, and since he was able to do it before, he would know that he's not cut out for it this time.
Another way of looking at is it that Tirek's Mana Drain only lets him absorb the raw energy of the person he drains. The bell, however, steals not only a person's inherent magical energy, but also the unique powers attached to that energy. When he first drained Discord, Tirek only received a boost to his own power, whereas if he tried to absorb the magic using the bell he'd be overwhelmed by the chaos energy attached to it.
Tempest Shadow is finally shown to have controlled her magic and has it acting like the others next to her. She's smiling because not only is she one of them, but she's also a true unicorn now with a working horn. She might not physically have the rest of her horn back yet, but at least her horn's magical ability returned.
Why would Discord suggest turning the villains to stone when he said that was a line he wouldn't cross in his debut appearance? Wouldn't that make him a huge Hypocrite? The answer is simple: it was already a hypocritical statement to begin with, since what he was doing to the ponies of Equestria was no better than the punishment he got for it, which he felt he didn't deserve. But now he feels he did deserve it, and since the villains have proven themselves to be just as bad as he was with no signs of remorse, he decides that they deserve it, too.
Twilight's turn of phrase "rainbow lasers" initially comes off as Schizo Tech since Equestria doesn't have actual lasers. But remember that Twilight has been to the Equestria Girls world a few times which has contemporary tech, and human Rainbow Dash explicitly calls their harmony beams "rainbow lasers" multiple times in the Equestria Girls franchise, including this moment when (pony-become-human) Twilight was right next to her:
Rainbow Dash: Uh, weren't there rainbows and lasers and stuff last time?
